This invention relates to a method for transferring an easily-polymerizable substance, and more particularly to a method for transferring an easily-polymerizable substance such as (meth)acrylic acid in a stable manner for a longer time by effectively preventing polymerization.
In the field of chemical industries, it has been strongly demanded to transfer an easily-polymerizable substance without polymerization in transfer passages. Many studies and trials have been made to satisfy this demand. Some proposed transferring manners and methods could successfully solve some problems. However, there yet remains problems to be solved.
As one of the problems to be solved, there is the problem that polymerization occurs in a pump which provides a moving force to an easily-polymerizable substance. Recently, leak-free pumps are used for transferring an easily-polymerizable substance as well as other liquids to securely preventing leakage of liquid which is harmful to human beings, explosive or flammable, or need careful handling.
A circulation type canned motor pump is one example of such leak-free pumps, and is provided with a circulation tube for circulating a part of liquid being transferred into a motor portion to cool the motor. In conventional circulation type canned motor pumps, a main shaft mounted with a rotor and an impeller is arranged along a horizontal axis, and an impeller of the pump is rotated about the horizontal axis. Circulated cooling liquid is flowed in the horizontal direction. With this construction, it is likely that the cooling liquid comes into sufficient contact with a lower part of bearings for rotatably supporting the rotor shaft, but comes into insufficient contact with an upper part of the bearings. Consequently, polymerization is liable to occur on the upper part of the bearings due to the heat generated by the motor in the case where the liquid being transferred is an easily-polymerizable substance or contains an easily adhesive substance.
With this construction in which the main shaft extends in the horizontal direction, the circulation tube becomes long in the horizontal direction. Accordingly, polymerization is liable to occur in the circulation tube due to the small sectional area and the long horizontal length of the circulation tube.
Polymerization on the bearing or its near portion and in the circulation tube causes obstruction for the circulation of cooling liquid, resulting in an undesirable rise in the temperature of the motor which causes further polymerization.
To overcome these problems, for example, Japanese Unexamined Patent Publication No. 6-272688 proposes polishing bearing contact portions of a rotor shaft of a motor with electrolysis to ensure smooth flow of cooling liquid around the bearings. However, the main shaft extends in the horizontal direction. Accordingly, this conventional canned motor pump can not entirely overcome the above-mentioned problem due to the horizontal shaft construction.
Also, Japanese Unexamined Patent Publication No. 8-73398 is directed to a circulation type canned motor pump for transferring an easily-polymerizable substance under a reduced pressure. In this canned motor pump, a part of circulated cooling liquid is passed through a chamber containing gas having oxygen and having a normal pressure, and then flowed into the motor portion. However, this canned motor pump has the main shaft extending in the horizontal direction, and accordingly can not sufficiently overcome the above-mentioned problem due to the horizontal shaft construction.
Formation of polymerized matter in a transferring pump or clogging or choking in a circulation tube of a circulation type canned pump, nevertheless, cannot be avoided when a chemical process system provided with such transferring pumps is operated for a long time in the aforementioned conventional manner because the material to be transferred is an easily-polymerizable compound such as (meth)acrylic acid. When choking or clogging occurs, it is required to suspend the operation of the system so as to chemically or manually remove the clogged polymerized matter. In a worse case, replacement of the pump is necessary.
It is an object of the present invention to provide an easily-polymerizable substance transferring method which is free from the problems residing in the prior art.
According to an aspect of the invention, an easily-polymerizable substance is transferred by a pump provided with a motor. A main shaft of the motor is arranged in a vertical direction. The main shaft is mounted with an impeller for imparting a moving force to an easily-polymerizable substance and a rotor constituting a part of the motor. The impeller is arranged in a lower portion while the rotor is arranged in an upper portion.
The arrangement of the main shaft carrying the impeller and rotor in the vertical direction will increase the motor cooling efficiency without causing polymerization, thereby ensuring the prolonged operation of the chemical processing system using an easily-polymerizable compound.
Other objects, features and advantages of the present invention will become apparent from the detailed description of the preferred embodiments of the invention to follow.